This invention relates to a multi-stage gas generator that utilizes an improved gas generant formulation. Gas generators (also known as inflators) have numerous applications in the commercial and consumer market. For example, they are used to deploy airbags used in automobiles, inflate floatation devices, and can be used in oxygen generating devices. Gas generators operate by burning a propellant contained therein extremely rapidly, usually in the millisecond range. For example, propellant is burned in side airbags in approximately 6-15 milliseconds; in passenger airbags in approximately 40-100 milliseconds; and in driver airbags in approximately 25-40 milliseconds. Most of the discussion in this application relates to automobile airbags, but similar principles apply to all types of inflators.
Until now, gas generators have burned the propellant in automobile airbags in one stage, causing, in some cases, injury to some adults and to children.
The propellants used in airbags have generally contained sodium azide, which, upon ignition, yielded particulates, including hot metallic oxides, and corrosive products, thus requiring expensive filtering systems to be certain these products do not harm the occupants. Alternative propellants have produced high temperature effluent and/or NOx gases, which also have required systems to protect the occupants of the car. Such airbag systems have also required various protective coatings, in order to prevent damage to the bags caused by the harmful by-products of combustion.
To date, uniform and reliable gas generation, for the systems indicated above, has been difficult to achieve, both mechanically (with respect to gas generation rate or slope) as well as chemically (with respect to control of the solid particulates and effluent resulting from propellant combustion). To date, at least one or more of the components (e.g., oxidizers) of the gas generants have been metal-based, leading to the formation of hot metallic solids or particulates as byproducts of combustion. The major airbag manufacturers still continue to use sodium azide (NaN3) as the main fuel constituent in their gas generant formulation and metallic oxides (e.g., copper oxide, iron oxide, molybdenum trioxide) as major oxidizer constituents in their formulations. These formulations generate, upon combustion, very hot copper-based, iron-based, or molybdenum-based solid byproducts, as well as NO, NO2, SO2, CO, and CO2, which have levels strictly enforced by the American Conference of Governmental Industrial Hygienists, and which many times can escape controls. Some of these combustion byproducts are extremely toxic to humans and are of great concern, despite the assurances of manufacturers that current gas generators produce such gas generant byproducts in only small quantities.
Some airbag systems have been based on propellants aside from sodium azide (see, for example, U.S. Pat. No. 5,482,579, where cellulose acetate, perchlorate and a metal oxide were used). However, these systems still generate hot metal particles or toxic or hot gases that require a filtration system to prevent harm to either the occupants or the airbags.
Other variations in airbags have been explored. For example, some use a mechanical means to control airflow in airbags. (See, for example, U.S. Pat. No. 6,050,601.) Mechanical means are, however, relatively slow compared to the extremely fast inflation required in airbags. Others have used systems that rely on stored gas for inflation. (See, for example U.S. Pat. No. 6,089,597.) Because of the difficulty in maintaining stored gas for long periods of time, these systems have not been widely used.
U.S. Pat. No. 5,876,062 relies on using a resistance wire to ignite the propellant. Vibration of the airbag system will cause the ignition wire to break, leading to malfunction of the system. Furthermore, a filtration system is also required. U.S. Pat. No. 6,199,906 relies on electronic logic to determine the extent to which the airbag is deployed. However, the system still generates noxious effluent and attempts to eliminate them through certain gas ports. Furthermore, the system recognizes that there may be accidental ignition of some portions of the system when exposed to heat or fire.